Electrophoretic display with dual mode switching

ABSTRACT

The present invention relates to an improved EPD which comprises both the traditional up/down switching and the in-plane switching modes. The monochrome EPDs of the present invention are capable of displaying highlight color of choice which is different from the text. For example, white background, blue text, and red highlight can be shown in any selected areas of the display. Furthermore, the full color EPDs of the present invention are capable of displaying high contrast images of high color saturation. Both high quality black and white states are possible in the full color displays of the present invention. The EPDs of the present invention do not need complex circuitry design, and are compatible with low cost and high yield roll-to-roll manufacturing processes. The EPD cells of the present invention may have opaque partition walls, or a black matrix top surface of the partition walls or a combination thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of U.S. patent applicationSer. No. 10/222,036 filed on Aug. 16, 2002, the content of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon influencing charged pigment particlessuspended in a solvent. This general type of display was first proposedin 1969. An EPD typically comprises a pair of opposed, spaced-apart andplate-like electrodes, with spacers predetermining a certain distancebetween the electrodes. At least one of the electrodes, typically on theviewing side, is transparent. For the passive type of EPDs, row andcolumn electrodes on the top (the viewing side) and bottom platesrespectively are needed to drive the displays. In contrast, an array ofthin film transistors (TFT) on the bottom plate and a non-patternedtransparent conductor plate on the top viewing substrate are requiredfor the active matrix type EPDs. An electrophoretic fluid composed of acolored dielectric solvent and charged pigment particles dispersedtherein is enclosed between the two electrodes.

When a voltage difference is imposed between the two electrodes, thepigment particles migrate by attraction to the plate of polarityopposite that of the pigment particles. Thus, the color showing at thetransparent plate may be determined by selectively charging the platesto be either the color of the solvent or the color of the pigmentparticles. Reversal of plate polarity will cause the particles tomigrate back to the opposite plate, thereby reversing the color.Intermediate color density (or shades of gray) due to an intermediatelevel of pigment particles attracted to the transparent plate may beobtained by controlling the plate charge through a range of voltages. Nobacklight is needed in this type of reflective EPD display, although itmay be optionally added to improve the display viewability in the dark.

EPDs of different pixel or cell structures have been reportedpreviously, for example, the partition-type EPD (see M. A. Hopper and V.Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)) and themicroencapsulated EPD (as described in U.S. Pat. No. 5,961,804 and U.S.Pat. No. 5,930,026). Each of these has its own problems as noted below.

In a partition-type EPD, there are partitions between the two electrodesfor dividing the space into smaller cells in order to prevent undesiredmovements of the particles such as sedimentation. However, difficultiesare encountered in the formation of the partitions, the process offilling the display with the fluid, enclosing the fluid in the display,and keeping the suspensions of different colors separated from eachother. Even more difficult problems are encountered in the developmentof a roll-to roll manufacturing process for such a partition type ofdisplays.

The microencapsulated EPD has a substantially two dimensionalarrangement of microcapsules each having therein an electrophoreticcomposition of a dielectric fluid and a dispersion of charged pigmentparticles that visually contrast with the dielectric solvent. Themicrocapsules are typically prepared in an aqueous solution, and toachieve a useful contrast ratio, their mean particle size is relativelylarge (50–150 microns). The large microcapsule size results in a poorscratch resistance and a slow response time for a given voltage becausea large gap between the two opposite electrodes is required for largecapsules. Also, the hydrophilic shell of microcapsules prepared in anaqueous solution typically results in sensitivity to high moisture andtemperature conditions. If the microcapsules are embedded in a largequantity of a polymer matrix to obviate these shortcomings, the use ofthe matrix results in an even slower response time and/or a lowercontrast ratio. To improve the switching rate, a charge-controllingagent is often needed in this type of EPDs. However, themicroencapsulation process in aqueous solution imposes a limitation onthe type of charge-controlling agents that can be used. Other drawbacksassociated with the microcapsule system include poor resolution and pooraddressability for color applications.

U.S. Pat. No. 3,612,758 discloses another type of EPDs wherein theelectrophoretic cells are formed from parallel line reservoirscontaining charged pigment particles dispersed in a dielectric solvent.The channel-like electrophoretic cells are covered with, and in electriccontact with, transparent conductors. A layer of transparent glass fromwhich side the panel is viewed overlies the transparent conductors.

The use of microchannels, microgrooves or microcolumns to form the EPDarray still has problem of undesirable particle sedimentation orcreaming along the column direction. In addition, the lack of aseamless, air-pocket free and continuous sealing process to enclose theelectrophoretic fluid in between the two electrodes makes theroll-to-roll manufacturing extremely difficult.

An improved EPD technology and a roll-to-roll manufacturing process wererecently disclosed in co-pending applications, U.S. Ser. No. 09/518,488,filed on Mar. 3, 2000, U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001,U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 and U.S. Ser. No.09/784,972, filed on Feb. 15, 2001, all of which are incorporated hereinby reference. The improved EPD comprises closed cells formed frommicrocups of well-defined shape, size and aspect ratio and filled withcharged pigment particles dispersed in a dielectric solvent andtop-sealed with a top-sealing composition by one of the methods asdisclosed in U.S. Ser. No. 09/518,488 (corresponding to WO 01/67170) andU.S. Ser. No. 09/874,391 (corresponding to WO02/98977), the contents ofwhich are incorporated herein by reference. This improved technologyinvolving microcups allows high image quality in monochrome EPDs. Acolor display may also be manufactured by using a spatially adjacentarray of small pixels formed of microcups filled with dyes of differentcolors (e.g., red, green or blue). However, there is a major deficiencyin this type of system with only the traditional up/down switching mode,that is, the white light reflected from the “turned-off” colored pixelsgreatly reduces the color saturation of the “turned-on” color. Moredetails in this regard are given in the following “Detailed Description”section.

While this latter deficiency may be remedied by an overlaid shutterdevice such as a polymer dispersed liquid crystal to switch the “off”pixels to the black state and keep the “on” pixels in the desired colorstate, the disadvantage of this approach is the high cost of theoverlaid device, the high Dmin (which is the minimum optical density ofthe background) associated with the shutter device, and the complicateddriving circuit design.

Alternatively, color displays of the normal up/down switching mode maybe prepared by using color filters overlaid on the viewing side of thedisplay. However, dark Dmin and lack of a high quality “white” state arethe major problems for reflective color displays using color filters.

The “in-plane” switching concept was disclosed in E. Kishi, et al.,“5.1: development of In-Plane EPD”, Canon Research Center, SID 00Digest, pages 24–27 and Sally A. Swanson, et al., “5.2: High PerformanceEPDs”, IBM Almaden Research Center, SID 00 Digest, pages 29–31. In theseprior art systems, the color effect is achieved by using a coloredbackground with white or black particles. The disadvantage of theseprior art systems is the lack of either high quality “white” or highquality “black” state. More details are also given in the “DetailedDescription” section.

Thus, there is still a need for an improved EPD with high quality fullcolor capability that can also be manufactured in an efficient manner,particularly by a roll-to-roll manufacturing process.

SUMMARY OF THE INVENTION

The present invention relates to an improved EPD which comprises boththe traditional up/down switching and the in-plane switching modes. Inother words, the improved EPD has dual switching modes.

The monochrome EPDs of the present invention are capable of displayinghighlight color of choice which is different from the color of the text.For example, white background, blue text, and red highlight can be shownin any selected areas of the display. Furthermore, the full color EPDsof the present invention are capable of displaying high contrast imagesof high color saturation. Both high quality black and white states arepossible in the full color displays of the present invention. The EPDsof the present invention do not need complex circuitry design, and arecompatible with low cost and high yield roll-to-roll manufacturingprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

It is noted that all figures are shown as schematic and are not toscale.

FIG. 1 illustrates the common deficiency of the traditional EPDs withonly the up/down switching mode.

FIGS. 2A–2D illustrate the lack of true white or true black state in anEPD with only the in-plane switching mode.

FIG. 3 illustrates a typical electrophoretic cell of the presentinvention and the general locations of the up/down and in-planeswitching electrodes.

FIGS. 4A–4C illustrate the various possible scenarios of the improvedEPD with dual modes.

FIG. 4D illustrates the highlight option of the present invention (topview).

FIGS. 5A and 5B illustrate the manufacture of microcups involvingimagewise photolithographic exposure through photomask.

FIGS. 6A and 6B illustrate the two-layered ITO electrode system.

FIGS. 7A and 7B illustrate the true white state of the presentinvention, the top and cross-section views.

FIGS. 8A and 8B illustrate the true black state of the presentinvention, the top and cross-section views.

FIGS. 9A and 9B illustrate the multiple color state of the presentinvention, the top and cross-section views.

FIGS. 10A–10E illustrate the TFT active driving mechanism.

FIGS. 11A–11E illustrate the combination of the active and passivedriving mechanisms.

FIGS. 12A–12E illustrate an alternative combination of the active andpassive driving mechanisms.

FIGS. 13A and 13B illustrate microcups having transparent and opaquepartition walls, respectively. FIG. 13C illustrates the two sides of apartition wall having different colors.

FIGS. 14A–14H illustrate the cross-section views of various black matrixlayer locations. FIG. 14I illustrates the top view of microcups havingpartition walls with a black matrix layer.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise in this specification, all technical terms areused herein according to their conventional definitions as they arecommonly used and understood by those of ordinary skill in the art. Theterms “well-defined”, “aspect ratio” and “imagewise exposure” are asdefined in the co-pending applications identified above.

It is understood that the scope of the present invention encompasses theconventional EPDs and EPDs manufactured from microcups, microchannels,microcapsules and the like.

The term “conventional EPD” refers to any electrophoretic cells known inthe art. The electrophoretic cells may be of any shapes and sizes, andthe displays include, for example, the partition type displays.

The term “microchannel” refers to the type of electrophoretic cellsdisclosed in U.S. Pat. No. 3,612,758.

The term “microcup” refers to the cup-like indentations, which may becreated by methods such as microembossing or imagewise exposure followedby a development step to remove the unexposed or exposed areas.Likewise, the plural form “microcups” in a collective context may ingeneral refers to the microcup assembly comprising a plurality of suchmicrocups integrally formed or joined to make a structuredtwo-dimensional microcup array. The dimensions of the microcup aredisclosed in the co-pending applications identified above.

The term “top-sealing” is intended to refer to a sealing process inwhich the display cells constructed on a first substrate or electrodelayer are filled and top-sealed. In the conventional edge seal process,two substrates or electrode layers and an edge seal adhesive arerequired to enclose and edge-seal the display fluid in the cell(s). Incontrast, in the top-sealing process, the display fluid is enclosed andtop-sealed before a second substrate or electrode layer is disposed ontothe display cell(s).

The term “Dmax” refers to the maximum achievable optical density of thedisplay.

The term “Dmin” refers to the minimum optical density of the displaybackground.

The term “contrast ratio” is defined as the ratio of the % reflectanceof an electrophoretic display at the Dmin state to the % reflectance ofthe display at the Dmax state.

I. The Disadvantages of Conventional EPDs

(1) EPD with Only Up/Down Switching

The EPD of FIG. 1 has only the up/down switching mode. The cells in thefigure are filled with a suspension in which white positively chargedparticles are dispersed in a colored (red, green and blue) dielectricfluid. All three cells in FIG. 1 are shown charged with a voltagedifference between the top and bottom electrodes (not shown). In thegreen and blue cells, the top electrode has a low voltage, the whitepositively charged particles in these two cells migrate to the topviewing electrode which is transparent, and as a result, the color ofthe particles (i.e., white) is reflected to the viewer through thetransparent conductor film in the two cells. In the red cell, the bottomelectrode has a low voltage; consequently the white positively chargedparticles migrate to the bottom of the cell, and the color of the medium(i.e., red) is seen through the top transparent conductor film. In thescenario as shown in FIG. 1, the white light reflected from the greenand blue pixels greatly reduces the color saturation of the red pixel.

(2) EPD with Only the In-Plane Switching Mode

FIGS. 2A–2D illustrates the disadvantages of the prior art EPDs withonly the in-plane switching mode.

In FIG. 2A, the cells are filled with a colorless dielectric solventwith white charged particles dispersed therein. The background of thecells is colored (i.e., red, green or blue). When there is a voltagedifference between the in-plane electrodes, the white particles migrateto either side of the cell, and the color of the background (i.e., red,green or blue) is seen from the top transparent opening. When there isno voltage difference between the in-plane electrodes, the particles arescattered in the dielectric solvent, resulting in white color (i.e., thecolor of the particles) being seen from the top transparent opening.This arrangement of colorless solvent, colored background and whiteparticles results in a display lack of a high density black state.

In FIG. 2B, the cells are filled with a colorless fluid with blackparticles dispersed therein. The background of the cells is colored(i.e., red, green or blue). When there is a voltage difference betweenthe in-plane electrodes, the particles migrate to either side of thecell, and the color of the background (i.e., red, green or blue) is seenfrom the top transparent opening. When there is no voltage differencebetween the in-plane electrodes, the particles are scattered in thedielectric solvent, resulting in a black color (i.e., the color of theparticles) being seen from the top transparent opening. This arrangementof solvent/background/particle colors results in a dirty white statewith undesirable Dmin and contrast ratio.

FIG. 2C shows the cells filled with a colorless fluid with coloredparticles (i.e., red, green or blue) dispersed therein. The backgroundof the cells is black. When there is a voltage difference between thein-plane electrodes, the colored charged particles migrate to eitherside of the cell, and the color of the background (i.e., black) is seenfrom the top transparent opening. When there is no voltage differencebetween the in-plane electrodes, the colored particles are scattered inthe dielectric solvent, resulting in the color of the particles (i.e.,red, green or blue) being seen from the top transparent opening. In thisdesign, the black state is of high quality. However, no high qualitywhite state is possible. As a result, the reflective display of thistype appears to have a dirty background or a low degree of reflection inthe Dmin area.

In FIG. 2D, the cells are filled with a colorless fluid with coloredparticles (red, green or blue) dispersed therein. The background of thecells is white. When there is a voltage difference between the in-planeelectrodes, the particles migrate to either side of the cell, and thecolor of the background (i.e., white) is seen from the top transparentopening, resulting in a high quality white state. When there is novoltage difference between the in-plane electrodes, the particles arescattered in the fluid, resulting in the color of the particles (i.e.,red, green or blue) being seen from the top transparent opening. No highquality black state is available in this design.

In summary, the in-plane only switching mode results in either areflective color display having no high quality black state or a displayhaving no high quality white state. Contrast ratio and color saturationare poor in this type of in-plane switching, reflective color displays.In all in-plane switching EPDs, the substrate on the opposite side ofthe in-plane electrodes is typically a transparent insulator, whichusually is the viewing side of the display.

II. Electrophoretic Display of the Present Invention

FIG. 3 illustrates a side view of a typical electrophoretic cell of thepresent invention. While only a cup-like cell is depicted, it isunderstood that the scope of the present invention encompasses cellsformed from microchannels and the like, and all types of conventionalelectrophoretic cells.

The cell (30) is sandwiched between a top (31) and a bottom layer (32).The top layer contains a transparent top electrode (not shown). Thebottom layer (32) consists of a layer (32 a) comprising an in-planeswitching electrode (34) on the left-hand side, a bottom electrode (35)and another in-plane electrode (36) on the right-hand side andoptionally a colored background layer (32 b). There is a gap (37) toseparate the two in-plane electrodes (34, 36) from the bottom electrode(35).

The background layer (32 b) may be on top of the electrode layer (32 a)(not shown), or underneath the electrode layer (32 a). Alternatively,the layer 32 a may serve as the background layer and in this case, thelayer 32 a may be black or of other colors.

Another alternative is that the bottom layer may have only one in-planeswitching electrode, and one bottom electrode with a gap in between.

Typically, the cells in FIG. 3 are filled with a clear, but colored(i.e., red, green or blue) dielectric solvent (38) with white particles(39) dispersed therein, and the background color of the cells istypically black. The particles may be positively or negatively charged.For the purpose of illustration, it is assumed that the particles arepositively charged throughout this application.

The charged particles in the individual cells of a display may be of thesame color or of different colors. The individual cells may also befilled with an electrophoretic fluid containing charged particles ofmixed colors.

As shown in FIGS. 4A–4C, the dual switching mode allows the particles tomove in either the vertical (up/down) direction or the planar(left/right) direction. For example, in FIG. 4A, the voltage of the topelectrode is set low, and the voltages of the bottom electrode and thein-plane electrodes are set high. The white particles migrate to andgather at the top transparent conductor film, and the white color (i.e.,the color of the particles) is seen by the viewer.

In FIG. 4B, the in-plane electrodes are set at low voltages, and the topand the bottom electrodes are set at high voltages. In this scenario,the white particles migrate to the sides of the cells, the color seenthrough the top transparent conductor film therefore is the color of thebackground (i.e., black).

In FIG. 4C, when the voltage of the top electrode is set high, thevoltage of the bottom electrode is set low, and the in-plane electrodesare set at low voltage, the white particles migrate to the bottom of thecells. In this scenario, the color of the fluid (i.e., red, green orblue) is seen by the viewer through the top transparent conductor filmas shown in the red cell of FIG. 4C. To present a red pixel in a fullcolor display, the white particles in the green and blue cells may beeither attracted to the sides as shown in FIG. 4C or to the top (notshown). The former is preferred because it typically exhibits a bettercolor saturation than the latter. Thus the dual switching modetechnology gives the first full-color EPD wherein all colors includingred, green, blue, black and white of high quality are available in thesame device.

Furthermore, the background color may be of any color (e.g., cyan,yellow or magenta) instead of the commonly used black color. Forexample, the cells of FIG. 3 may be filled with a red clear dielectricsolvent with white positively charged particles dispersed therein andthe background color of the cells may be yellow. In this case, when theparticles migrate to the top, the white color (i.e., the color of theparticles) is seen by the viewer and when the particles migrate to coverthe bottom of the cells, the color of the medium (i.e., red) is seenthrough the transparent conductor. However, when the white particlesmigrate to the sides of the cells, the color seen through the toptransparent conductor film, will be a shade of orange.

Other shades or color tones may be achieved by using differentparticle/medium/background color combinations, for example,white/red/cyan, white/red/magenta, white/blue/yellow, white/blue/cyan,white/blue/magenta, white/green/yellow, white/green/cyan,white/blue/magenta, etc.

The preferred combination to achieve a full color display is whiteparticles, black background, and fluids separately colored with anadditive primary color (i.e., red, green or blue).

A further aspect of the invention is a monochrome display with highlightoptions. In such a case, all cells in the display have the samebackground color and are filled with the same electrophoretic fluid(i.e., having the same particle/solvent color combination). For example,the display may have white particles, the solvent is one of the primarycolors (red, green or blue) and the background color is a colorcontrasting the solvent color. This arrangement is useful for arelatively simple two color device with a colored highlight option. Forexample, an EPD having white particles, a yellow dielectric solvent, anda black background can display at least three different colors in eachcell or pixel as shown in FIG. 4D (top view). When the white particlesare all attracted to the top viewing electrode, the white color is seen.When the white particles are uniformly attracted to the bottomelectrodes, the yellow color is seen through the top viewing electrode.When the white particles are attracted to the in-plane electrode oneither side of the cell, the black color is seen through the top viewingelectrode. Intermediate colors are also possible if the particles aredriven to intermediate states. This highly desirable feature ofhighlight capability in any pixels of the display can be realized byusing the dual switching mechanism of this invention to drive a low costmonochrome EPD having pre-selected colors for the particles, the solventand the background.

In summary, the EPDs of the present invention with the dual switchingmode can provide the previously unattainable high quality full colorEPDs and a monochrome EPD with highlight color capability in any pixelsof a monochrome display.

III. Preparation of Electrophoretic Cells of the Present Invention

The microcups generally may be manufactured by microembossing orphotolithography as disclosed in U.S. Ser. No. 09/518,488 filed Mar. 3,2000 and U.S. Ser. No. 09/784,972 filed on Feb. 15, 2001.

While only the cup-like cells are illustrated in the figures, it isunderstood that conventional electrophoretic cells and electrophoreticcells prepared from microchannels, microcolumns and the like are alsowithin the scope of the present invention.

III(a) Preparation of the Microcup Array by Microembossing Preparationof the Male Mold

The male mold may be prepared by any appropriate method, such as adiamond turn process or a photoresist process followed by either etchingor electroplating after the resist is developed. A master template forthe male mold may be manufactured by any appropriate method, such aselectroplating. With electroplating, a glass base is sputtered with athin layer (typically 3000 Å) of a seed metal such as chrome inconel. Itis then coated with a layer of photoresist and exposed to UV. A mask isplaced between the UV and the layer of photoresist. The exposed areas ofthe photoresist become hardened. The unexposed areas are then removed bywashing them with an appropriate solvent. The remaining hardenedphotoresist is dried and sputtered again with a thin layer of seedmetal. The master is then ready for electroforming. A typical materialused for electroforming is nickel cobalt. Alternatively, the master canbe made of nickel by electroforming or electroless nickel deposition asdescribed in “Continuous manufacturing of thin cover sheet opticalmedia”, SPIE Proc. 1663:324 (1992). The floor of the mold is typicallybetween about 50 to 400 microns thick. The master can also be made usingother microengineering techniques including e-beam writing, dry etching,chemical etching, laser writing or laser interference as described in“Replication techniques for micro-optics”, SPIE Proc. 3099:76–82 (1997).Alternatively, the mold can be made by diamond turning or photomachiningusing plastics, ceramics or metals.

The male mold thus prepared typically has protrusions between about 3 to500 microns, preferably between about 5 to 100 microns, and mostpreferably about 10 to 50 microns, and can be of any shape like round,square, or of other geometry. The male mold may be in the form of abelt, a roller, or a sheet. For continuous manufacturing, the belt orthe roller type of mold is preferred. Prior to applying a UV curableresin composition, the mold may be treated with a mold release to aid inthe demolding process. To further improve the demolding process, theconductor film may be precoated with a primer or an adhesion promotinglayer to improve the adhesion between the conductor and the microcups.

Microcups may be formed either in a batchwise process or in a continuousroll-to-roll process as described in U.S. Ser. No. 09/784,972 filed onFeb. 15, 2001, the content of which is incorporated herein by reference.

In the first step of the microembossing process, a UV curable resin istypically coated on a transparent patterned conductor film, by anyappropriate means, such as roller coating, die coating, slot coating,slit coating, doctor blade coating, and the like. The conductor film isusually prepared by sputtering coating on a plastic substrate such aspolyethylene terephthalate, polyethylene naphthate, polyaramid,polyimide, polycycloolefin, polysulfone and polycarbonate. The radiationcurable material used is a thermoplastic or thermoset precursor, such asmultifunctional acrylate or methacrylate, vinylether, epoxide and theiroligomers, polymers and the like. Multifunctional acrylates and theiroligomers are the most preferred. A combination of multifunctionalepoxide and multifunctional acrylate is also very useful to achievedesirable physico-mechanical properties. The UV curable resin may bedegassed prior to dispensing and may optionally contain a solvent. Thesolvent, if present, readily evaporates.

The radiation curable material coated on the conductor film/substrate isembossed by the male mold under pressure. If the male mold is metallicand opaque, the conductor film/substrate is typically transparent to theactinic radiation used to cure the resin. Conversely, the male mold canbe transparent and the conductor film/substrate can be opaque to theactinic radiation.

After exposure to radiation, the radiation curable material becomeshardened. The male mold is then released from the microcups formed.

III(b) Preparation of Microcup Array by Photolithography

The photolithographic process for preparation of the microcup array isshown in FIGS. 5A and 5B.

As shown in FIGS. 5A and 5B, the microcup array may be prepared byexposure of a radiation curable material (51 a), coated by any knownmethods onto a transparent patterned conductor film (52), to UV light(or alternatively other forms of radiation, electron beams and the like)through a mask (56) to form walls (51 b) corresponding to the imageprojected through the mask (56). The conductor film (52) is on a plasticsubstrate (53).

In the photomask (56) in FIG. 5A, the dark squares (54) represent thearea opaque to the radiation employed, and the space (55) between thedark squares represents the radiation-transparent area. The UV radiatesthrough the opening area (55) onto the radiation curable material (51a).

As shown in FIG. 5B, the exposed areas (51 b) become hardened and theunexposed areas (protected by the opaque area (54) of the mask (56)) arethen removed by an appropriate solvent or developer to form themicrocups (57). The solvent or developer is selected from those commonlyused for dissolving or dispersing radiation curable materials such asmethylethylketone, toluene, acetone, isopropanol or the like.

Alternatively, the exposure can be done by placing the photomaskunderneath the conductor film/substrate. In this case, the conductorfilm/substrate must be transparent to the radiation wavelength used forexposure.

III(c) Opaque Partition Walls

In one embodiment, the partition walls of the microcups are preferablyopaque (such as white opaque or gray opaque). FIGS. 13A and 13B aresimplified cross-section views of the microcups having transparent oropaque partition walls, respectively. FIG. 13A shows that theelectrophoretic composition in one microcup may be seen from the top ofa neighboring microcup through the transparent partition wall dividingthe two microcups. In such a case, more than one color may be perceivedby the viewer which results in a color shift or parallax (double image).This phenomenon may be avoided if the partition wall is opaque as shownin FIG. 13B. The opaque partition walls may be achieved by adding afiller material such as silica, ZnO, TiO₂, BaSO₄, CaCO₃ or polymerparticles, preferably silica or polymer particles in the amount of 1–20%by weight, preferably in the amount of 2–10% by weight, into theradiation curable material for the formation of the microcups asdescribed above.

The partition walls may also be colored by a dye or pigment in theradiation curable material for the formation of the microcups. Thefiller, dye or pigment used in the radiation curable composition shouldnot interfere with the polymerization or crosslinking of the compositionduring the microcup forming process. The maximum concentration of thefiller, dye or pigment in the radiation curable microcup formulation isdependent on the optical density of the composition and the type ofradiation used to cure the composition. To facilitate the microcupformation and mold release, the resultant radiation curable compositionmust allow enough light or radiation to reach the bottom of themicrocups to assure proper physicomechanical properties of the microcupsand their adhesion to the substrate underneath.

A latent light scattering material may be used to produce the opaquepartition walls. Light scattering centers such as air pockets ordispersed polymer phase may be produced from the latent material duringor after the microcup forming step(s). For example, a filler materialthat is marginally compatible with the radiation curable composition butincompatible with the cured microcups may be added in the radiationcurable composition. Upon exposure to radiation, the filler materialphase-separates and forms discrete light-scattering domains in the curedmicrocup structure. Similarly, air pockets may be formed during or afterthe microcup forming step(s) by incorporating, into the radiationcurable microcup composition, a thermally or photochemically triggeredgas-releasing material as described in G. J. Sabongi, “ChemicalTriggering, Reactions of Potentially Utility in Industrial Processes”,Plenum Press, (1987). Examples of the gas releasing reactions mayinclude CO₂ from carboxylic acids, bicyclic lactones or heterocycles; COfrom ketones or bicyclic adducts; SO and SO₂ from sulfones, sulfonyloxycompounds or heterocycles, particularly 3-membered and 5-memberedheterocycles; N₂ from azoalkanes, azides, diazomethanes, N-nitrosocompounds, diazo, triazo or tetraazo heterocycles; COS from xanthates;or O₂ from endoperoxides. Air pockets may also be formed by anultrasonic-wave triggered gas release reaction of phenol, tropolone,pyridine, pyrazine, pyrrole or halogenated derivatives thereof.

In one embodiment, the inside surface (130) of the partition walls (131)may be colored to match the color of the dielectric solvent in theelectrophoretic composition as shown in FIG. 13C. In this scenario, itis possible that the two sides of a partition wall may have twodifferent colors if the two adjacent electrophoretic cells are ofdifferent colors. For example, if cell A in FIG. 13C is filled with ared electrophoretic composition whereas the adjacent cell B is filledwith a green electrophoretic composition, one side (130 a) of thepartition wall facing the red cell may be of the red color and the otherside (130 b) facing the green cell may be of the green color. Thedifferent sides of partition walls may be colored sequentially by, forexample, impregnating selective microcups in a dye or pigmentsolution/dispersion before the selective filling/sealing steps offorming a color display according the process disclosed in copendingU.S. patent application Ser. No. 09/879,408, filed on Jun. 11, 2001, thecontent of which is incorporated herein by reference.

The openings of the microcups prepared according to the methodsdescribed above may be round, square, rectangular, hexagonal, or anyother shape. The partition area between the openings is preferably keptsmall in order to achieve high color saturation and contrast ratio whilemaintaining desirable mechanical properties. Consequently, thehoneycomb-shaped opening is preferred over, for example, the circularopening.

For reflective electrophoretic displays, the dimension of eachindividual microcup may be in the range of about 10² to about 1×10⁶ μm²,preferably from about 10³ to about 1×10 ⁵ μm². The depth of themicrocups is in the range of about 5 to about 200 microns, preferablyfrom about 10 to about 100 microns. The opening to the total area ratio,total area being defined as that of one cup including walls measuredfrom wall centers, is in the range of from about 0.05 to about 0.95,preferably from about 0.4 to about 0.9.

The colored background layer of the cells may be added by painting,printing, coating, vapor deposition, sputtering or laminating a coloredlayer onto the bottom layer (the non-viewing side).

III(d) Preparation of the Suspensions

The cells are filled with an electrophoretic composition comprisingcharged pigment particles dispersed in a colored dielectric solvent. Theelectrophoretic composition may optionally contain additional colorantsthat do not migrate in the electric field. The pigment dispersion may beprepared according to methods well known in the art, such as U.S. Pat.Nos. 6,017,584, 5,914,806, 5,573,711, 5,403,518, 5,380,362, 4,680,103,4,285,801, 4,093,534,4,071,430, and 3,668,106. See also IEEE Trans.Electron Devices, ED-24, 827 (1977), and J. Appl. Phys. 49(9):4820(1978).

The dielectric solvent preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvents include hydrocarbons such as decahydronaphthalene (DECALIN),5-ethylidene-2-norbornene, fatty oils, paraffin oil, aromatichydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzeneand alkylnaphthalene, halogenated solvents such as,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride,chloropentafluoro-benzene, dichlorononane, pentachlorobenzene, andperfluorinated solvents such as perfluorodecalin, perfluorotoluene,perfluoroxylene, FC-43, FC-70 and FC-5060 from 3M Company, St. PaulMinn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylethers such asGalden, HT-200, and Fluorolink from Ausimont or Krytox Oils and GreasesK-Fluid Series from DuPont, Delaware. In one preferred embodiment,poly(chlorotrifluoroethylene) is used as the dielectric solvent. Inanother preferred embodiment, poly(perfluoropropylene oxide) is used asthe dielectric solvent.

The dielectric solvent may be colored by a contrasting dye or pigment.Nonionic azo and anthraquinone dyes are particularly useful. Examples ofuseful dyes include, but are not limited to: Oil Red EGN, Sudan Red,Sudan Blue, Oil Blue, Macrolex Blue, Solvent Blue 35, Pylam Spirit Blackand Fast Spirit Black from Pylam Products Co., Arizona, Sudan Black Bfrom Aldrich, Thermoplastic Black X-70 from BASF, anthraquinone blue,anthraquinone yellow 114, anthraquinone reds 111 and 135, anthraquinonegreen 28 from Aldrich. Fluorinated dyes are particularly useful whenperfluorinated solvents are used. In case of a contrasting pigment, thecolorant of the medium may also be dispersed in the dielectric mediumand are preferably uncharged. If the contrasting color pigment particlesare charged, they preferably carry a charge which is opposite from thatof the charged primary color pigment particles. If both the contrastingcolor and the primary color pigment particles carry the same charge,they should have different charge density or different electrophoreticmobility. The dyes or pigments used in EPDs must be chemically stableand compatible with other components in the suspension.

The charged primary color particles are preferably white, and may beorganic or inorganic pigments, such as TiO₂, BaSO₄ or ZnO.

If colored pigment particles are used, they may be selected fromphthalocyanine blue, phthalocyanine green, diarylide yellow, diarylideAAOT yellow, and quinacridone, azo, rhodamine, perylene pigment seriesfrom Sun Chemical, Hansa yellow G particles from Kanto Chemical, andCarbon Lampblack from Fisher. Particle size is preferably in the rangeof 0.01–5 microns, and is even more preferably in the range of 0.05–2microns. The particles should have acceptable optical characteristics,should not be swollen or softened by the dielectric solvent, and shouldbe chemically stable. The resulting suspension must also be stableagainst sedimentation, creaming or flocculation under normal operatingconditions.

The migrating pigment particles may exhibit a native charge, or may becharged explicitly using a charge control agent, or may acquire a chargewhen suspended in the dielectric solvent. Suitable charge control agentsare well known in the art; they may be polymeric or non-polymeric innature, and may also be ionic or non-ionic, including ionic surfactantssuch as Aerosol OT, sodium dodecylbenzenesulfonate, metal soaps,polybutene succinimide, maleic anhydride copolymers, vinylpyridinecopolymers, vinylpyrrolidone copolymer (such as Ganex from InternationalSpecialty Products), (meth)acrylic acid copolymers, andN,N-dimethylaminoethyl (meth)acrylate copolymers. Fluorosurfactants areparticularly useful as charge controlling agents in perfluorocarbonsolvents. These include FC fluorosurfactants such as FC-170C, FC-171,FC-176, FC430, FC431 and FC-740 from 3M Company and Zonylfluorosurfactants such as Zonyl FSA, FSE, FSN, FSN-100, FSO, FSO-100,FSD and UR from DuPont.

Suitable charged pigment dispersions may be manufactured by any of thewell-known methods including grinding, milling, attriting,microfluidizing, and ultrasonic techniques. For example, pigmentparticles in the form of a fine powder are added to the suspendingsolvent and the resulting mixture is ball milled or attrited for severalhours to break up the highly agglomerated dry pigment powder intoprimary particles. Although less preferred, a dye or pigment forgenerating color of the dielectric solvent may be added to thesuspension during the ball milling process.

Sedimentation or creaming of the pigment particles may be eliminated bymicroencapsulating the particles with suitable polymers to match thespecific gravity to that of the dielectric solvent. Microencapsulationor coating of the pigment particles may be accomplished chemically orphysically. Typical microencapsulation processes include interfacialpolymerization, in-situ polymerization, phase separation, coacervation,electrostatic coating, spray drying, fluidized bed coating and solventevaporation.

Density matched pigment-containing microparticles may be preparedaccording to methods disclosed in copending U.S. patent applications,U.S. Ser. No. 60/345,936, filed on Jan. 3, 2002, U.S. Ser. No.60/345,934 also filed on Jan. 3, 2002, U.S. Ser. No. 10/335,210 filed onDec. 31, 2002, U.S. Ser. No. 10/335,051 filed on Dec. 31, 2002, U.S.Ser. No. 60/400,021 filed on Jul. 30, 2002, U.S. Ser. No. 60/418,078filed on Oct. 10, 2002 and U.S. Ser. No. 60/356,226 filed on Feb. 11,2002, the contents of which are incorporated herein by reference intheir entirety. The average particle size of the density-matchedpigment-containing microparticles may be in the range of 0.1–10 μm,preferably in the range of 0.25–3 μm.

III(e) Filling and Sealing of the Microcups

The filling and sealing procedures are described in the co-pendingapplications, U.S. Ser. No. 09/518,488, U.S. Ser. No. 09/784,972, U.S.Ser. No. 09/879,408, U.S. Ser. No. 09/874,391 and U.S. Ser. No.60/408,256, the disclosures of which are incorporated herein byreference in their entirety.

After the microcups are filled with an electrophoretic composition, theyare top-sealed. The critical step of top-sealing of the microcups may beaccomplished in a number of ways. A preferred approach is to disperse aUV curable composition into an electrophoretic fluid comprising chargedpigment or pigment-containing particles dispersed in a coloreddielectric solvent. The suitable UV curable materials may includeacrylates, methacrylates, styrene, alpha-methylstyrene, butadiene,isoprene, allylacrylate, polyvalent acrylate or methacrylate,cyanoacrylates, polyvalent vinyl including vinylbenzene, vinylsilane,vinylether, polyvalent epoxide, polyvalent isocyanate, polyvalent allyl,and oligomers or polymers containing crosslinkable functional groups.The UV curable composition is immiscible with the dielectric solvent andhas a specific gravity lower than that of the electrophoreticcomposition comprising pigment particles dispersed in a dielectricsolvent. The two compositions, UV curable composition and theelectrophoretic composition, are thoroughly blended with, for example,an in-line mixer, and immediately coated onto the microcups with aprecision coating mechanism such as Myrad bar, gravure, doctor blade,slot coating or slit coating. Excess fluid is removed by a wiper bladeor a similar device. A small amount of a weak solvent or solvent mixturesuch as heptane, isopropanol and methanol may be used to clean theresidual electrophoretic fluid on the top surface of the partition wallsof the microcups. Volatile organic solvents may be used to control theviscosity and coverage of the electrophoretic fluid. The thus-filledmicrocups are then dried and the UV curable composition floats to thetop of the electrophoretic fluid. The microcups may be top-sealed bycuring the supernatant UV curable layer during or after it floats to thetop. The UV light or other forms of radiation such as visible light, IRor electron beam may be used to cure the top-sealing layer and seal themicrocups. Alternatively, heat or moisture may also be employed to curethe top-sealing layer and seal the microcups, if a heat or moisturecurable composition is used.

A preferred group of dielectric solvents exhibiting desirable densityand solubility discrimination against acrylate monomers and oligomersare halogenated hydrocarbons, particularly perfluoro solvents such asperfluoroethers from Ausimont, Italy or Du Pont, Delaware, and theirderivatives. Surfactants may be used to improve the adhesion and wettingat the interface between the electrophoretic fluid and the sealingmaterials. Surfactants include the FC surfactants from 3M Company, Zonylfluorosurfactants from DuPont, fluoroacrylates, fluoromethacrylates,fluoro-substituted long chain alcohols, perfluoro-substituted long chaincarboxylic acids and their derivatives.

Alternatively, the electrophoretic fluid and the top-sealing compositionmay be applied sequentially onto the microcups to prevent intermixing,particularly when the sealing precursor is at least partially compatiblewith the dielectric solvent. Thus, the top-sealing of the microcups maybe accomplished by overcoating a thin layer of a top-sealing compositionwhich is hardenable by radiation, heat, solvent evaporation, moisture orinterfacial reactions on the surface of the filled microcups. Volatileorganic solvents may be used to adjust the viscosity and the thicknessof the coatings. When a volatile solvent is used in the overcoat, it ispreferred that it is immiscible with the dielectric solvent to reducethe degree of intermixing between the top-sealing layer and theelectrophoretic fluid. To further reduce the degree of intermixing, itis highly desirable that the specific gravity of the overcoatedtop-sealing composition is no greater than that of the electrophoreticfluid. In the co-pending patent application, U.S. Ser. No. 09/874,391,filed Jun. 4, 2001, compositions comprising a thermoplastic elastomerhave been disclosed as the preferred top-sealing composition. Incopending patent application U.S. Ser. No. 60/408,256, the content ofwhich is incorporated herein by reference, compositions comprising apolyurethane have been disclosed as the preferred top-sealingcomposition. Additives such as silica particles, binder polymers orsurfactants may be used to improve the film integrity and coatingquality.

Alternatively, interfacial polymerization followed by UV curing has beenfound beneficial to the top-sealing process. Intermixing between theelectrophoretic layer and the top-sealing overcoat is significantlysuppressed by the formation of a thin barrier layer at the interface byinterfacial polymerization or crosslinking. The top-sealing is thencompleted by a post curing step, preferably by UV radiation. Thetwo-step overcoating process is particularly useful when the dye used isat least partially soluble in the top-sealing composition.

III(f) Lamination of the Microcups

The top-sealed microcups are then laminated with the other electrodefilm preferably with an adhesive layer. Suitable adhesive materialsinclude acrylic and rubber types of pressure sensitive adhesives, UVcurable adhesives containing for example, multifunctional acrylates,epoxides, vinylethers or thiolene, and moisture or heat curableadhesives such as epoxy, polyurethane, and cyanoacrylate.

In one embodiment of the invention, a substrate containing thin filmtransistors may be used as one of the bottom layer electrodes to alsoprovide the active driving mechanism and the top electrode, in thisscenario, is transparent.

The second (top) electrode layer may also be disposed on the top-sealedmicrocups by coating, printing, vapor deposition, sputtering or acombination thereof.

The top electrode layer may also be coated or laminated with a topsubstrate layer.

III(g) Application of a Black Matrix Layer Over the Top Surface of thePartition Walls

When the viewing side is from the top-sealing side of the microcups, theblack matrix may be applied between the top surface (140) of thepartition walls (141) and the top-sealing layer (142) as shown in FIG.14A, between the top-sealing layer (142) and the adhesive layer (143),if present, as shown in FIG. 14B, between the adhesive layer (143), ifpresent, and the top electrode layer (144) as shown in FIG. 14C, betweenthe top electrode layer (144) and the top substrate (145) as shown inFIG. 14D or on top of the top substrate (145) as shown in FIG. 14E. Whenthe viewing side is from the bottom, the black matrix may be appliedbetween the microcups (146) and the bottom electrode layer (147) asshown in FIG. 14F, between the bottom electrode layer (147) and thebottom substrate (148) as shown in FIG. 14G or on the bottom surface ofthe bottom substrate (148) as shown in FIG. 14H.

A black matrix layer may be applied with registration to the partitionwalls. In other words, the black matrix layer is on the top surface ofthe partition walls or on another layer in areas corresponding to thetop surface of the partition walls. The openings of the microcups orareas corresponding to the openings of the microcups are not covered bythe black matrix layer. The black matrix layer may be applied by amethod such as printing, stamping, photolithography, vapor deposition orsputtering with a shadow mask. The optical density of the black matrixmay be higher than 0.5, preferably higher than 1. Depending on thematerial of the black matrix and the process used to dispose the blackmatrix, the thickness of the black matrix may vary from 0.005 μm to 5μm, preferably from 0.01 μm to 2 μm.

In one embodiment as shown in FIG. 14A, a thin layer of black coating orink may be transferred onto the top surface (140) of the partition walls(141) after formation of the microcups, by an offset rubber roller orstamp. After the transferred coating or ink is hardened, the microcupsare subsequently filled and top-sealed (142). In this case, the hardenedblack matrix must be resistant to the solvent(s) used in both theelectrophoretic and top-sealing compositions.

In another embodiment as shown in FIG. 14B, the black matrix layer maybe applied, with registration to the partition walls, onto thetop-sealing layer after the microcups are filled and top-sealed. Forexample, a photosensitive black coating may be coated onto thetop-sealing layer (142) and exposed imagewise with registration througha photomask. The photosensitive black coating may be apositively-working or negatively-working resist. When apositively-working resist is used, the photomask should have openingscorresponding to the microcup areas. In this scenario, thephotosensitive black coating in the microcup areas (exposed) is removedby a developer after exposure. If a negatively-working resist is used,the photomask should have openings corresponding to the top surface ofthe partition walls. In this scenario, the photosensitive black coatingin the microcup areas (unexposed) is removed by a developer afterexposure. The solvent(s) used to apply the black coating and thedeveloper(s) for removing the coating should be carefully selected sothat they do not attack the top-sealing layer.

Alternatively, a colorless photosensitive ink-receptive layer may beapplied onto the top sealing layer followed by exposure through aphotomask. If a positively-working photosensitive latent ink-receptivelayer is used, the photomask should have openings corresponding to thetop surface of the partition walls. In this scenario, after exposure,the exposed areas become ink-receptive or tacky and a black matrix maybe formed on the exposed areas (the top surface of the partition walls)after a black ink or toner is applied onto those areas. Alternatively, anegatively-working photosensitive ink-receptive layer may be used. Inthis case, the photomask should have openings corresponding to themicrocups and after exposure, the exposed areas (the microcup areas) arehardened while a black matrix layer may be formed on the unexposed areas(the top surface of the partition walls) after a black ink or toner isapplied onto those areas. The black matrix may be post cured by heat orflood exposure to improve the film integrity and physicomechanicalproperties.

In another embodiment, the black matrix may be applied by printing suchas screen printing or offset printing, particularly waterless offsetprinting.

In still another embodiment, the black matrix may be applied onto theadhesive layer, if present, electrode layer or the substrate layer withregistration to the top surface of the partition walls by any of themethods mentioned above.

To complete the display assembly, a diffuser layer may be applieddirectly or indirectly above the black matrix layer to improve thevisual effect of the finished display device.

FIG. 14I is the top view of microcups having partition walls with ablack matrix top surface. As shown, the areas over the openings of themicrocups remain transparent. While microcups of a square opening areshown in FIG. 14I, the top opening shape of the microcups may vary. Itmay be rectangular, circular or a more complex shape if desired.

The black matrix layer added to the top surface of the color displaysignificantly improves the contrast ratio and color saturation of thedisplay. In the case when a highly crosslinked black matrix layer isformed on the top surface of the display from, for example, theabove-mentioned photosensitive ink-receptive layer, it may also renderthe top surface more resistant to scratch and finger prints.

While not specifically described, it is understood that the combinationof the black matrix layer as described in this section and the opaque orcolored partition walls as described above in Section III(c) above isalso beneficial.

IV. The Operations of the Dual Mode Switching

IV(a) Passive Matrix

(1) Electrode Circuit Design

FIG. 6A is the side view of the two-layer passive matrix electrodecircuit design. FIG. 6B shows the top view of a two-layer passive matrixelectrode design for dual-mode. The cells (60) are sandwiched betweenone top layer (61) and a bottom layer (62). The horizontal bars are therow electrodes (63) that are transparent and run through the top of thecells. The bottom layer (62) consists of one in-plane electrode (64)that is on the left-hand side of the cell, one bottom column electrode(65) and another in-plane electrode (66) on the right-hand side. Thereare gaps (67) between the in-plane electrodes and between the in-planeelectrodes and the column electrodes.

The cross section of the top row electrode, the bottom column electrode,and the in-plane electrodes define the display cell.

(2) Driving Timing Sequences

To illustrate the dual mode switching of this invention, electrophoreticcells comprising a common black background and positively charged whiteparticles dispersed in a clear colored solvent are used.

A true white state may be generated by applying the top row electrodes(63) of selected cells or pixels with a lower voltage and at the sametime applying the bottom column electrodes (65) and the in-planeelectrodes (64) with a higher voltage. At this bias setting, theparticles are driven to the top row electrodes, and the white color isseen through the top transparent conductor layer by the viewer.

FIG. 7A is the cross-sectional view of an array of cells, whichdemonstrates the true white state. When the white particles in all cellsmigrate to the top of the cells, the resulting color (i.e., white) isseen from the top through the transparent conductor film (not shown).FIG. 7B is the top view of the array of cells showing the true whitestate.

A true black state can be achieved by, for example, a two-step drivingprocess. In the first step, the voltage of the row electrode (63) is sethigh and the voltages of the column electrode (65) and the two in-planeelectrodes (64) are set low. As a result, white particles are firstattracted to the bottom of the cells. In the second step, the voltage ofthe in-plane electrodes is set low, the voltage of the column electrodeis set high, and the row electrode is also set high. Under thesesettings, the white particles, driven by the electric field, migrate toand cover the sides of the cells, resulting in the black backgroundcolor being seen through the top transparent conductor film.

Alternatively, a true black state can also be achieved by using aone-step driving process. More specifically, the black color may be seenby setting the row (63) and column (65) electrodes of the selected cellswith high voltages, and the in-plane electrodes (64) at a low voltage.The voltages applied to the row and column electrodes may not be thesame. This allows the electric field from both the top row electrode(63) and the bottom column electrode (65) to force the particles in theselected cells to move quickly towards the edges of the cells andresults in a true black state of high quality.

FIG. 8A is the cross-sectional view of the same array of cells (as wasshown in FIG. 7A) to demonstrate the true black state of this invention.The white particles in all cells migrate to the sides of the cells,resulting in the color of the background (i.e., black) being seen fromthe top transparent conductor film. FIG. 8B is the top view of the arrayof cells showing the true black state.

A colored (e.g., red, blue, or green) state of the selected cells can beachieved when the voltage of a top (row) electrode (63) is set high, andthe voltages of a column electrode (65) and the two in-plane electrodes(64) are set low. In this case, the white particles in the cell, drivenby the electric field, migrate to the column electrode at the bottom.The bottom of the cells will be covered by the white particles and thecolor of the dielectric solvent (i.e., red, green or blue) is seenthrough the top transparent conductor layer. Any color combinations maybe achieved according to the present invention by moving the chargedwhite particles in selected cells of a pixel to the bottom. Gray scalecan also be obtained by adjusting the voltage to partially move theparticles to the bottom column electrodes.

FIG. 9A is the cross-sectional view of the same array of cell (as shownin FIGS. 7A and 8A) to demonstrate the white, black and two color statesin the same EPD display of the present invention. The cells with thewhite particles migrated to the top row electrodes (63) show the whitecolor; the cells with the white particles migrated to bottom columnelectrodes (65) show the color of the dielectric solvent (i.e., red,green or blue); and the cells with the white particles migrated to thesides of the cells show the black color. FIG. 9B is the top view of thearray of cells showing multiple colors.

Although the dual switching mode was illustrated above by using twoin-plane electrodes (64) in a cell, the same results can also beachieved by using only one in-plane electrode or multiple in-planeelectrodes.

IV(b) TFT Active Matrix

(1) Electrode Circuit Design

TFT (Thin Film Transistor) Active Matrix used in the LCD display systemhas been disclosed in U.S. Pat. No. 5,132,820. Such TFT technology canalso be applied to the dual-mode EPD of the present invention. In apreferred embodiment, the top layer electrode made of a transparentconductive material, such as ITO, is in one piece which covers theentire top surface of the display. The top electrode connects to ground(0V). FIG. 10A shows the top view of the bottom electrode layer of a 2×2array of cells. FIG. 10 B shows the details of the TFT connection. Eachcell (100) comprises one bottom electrode (101) and two in-planeelectrodes (102). Each of the bottom electrodes connects to the drain(103 a) of a TFT (104 a). The in-plane electrodes of each cell connectto the drain (103 b) of another TFT (104 b). The sources (105 a and 105b) of the TFTs (104 a and 104 b) connect to signal lines (106 a and 106b) which run vertically through the bottom surface of the device. Thegates (107 a and 107 b) of the TFTs (104 a and 104 b) connect to a scanline (108), which runs horizontally through the bottom surface of thedevice. The scan and signal lines form a matrix structure, but they areinsulated from each other.

In the dual-mode switching, two TFTs (104 a and 104 b) are required foreach cell to control the bottom electrode (101) and the in-planeelectrodes (102) independently. When a row of cells (100) are scanned,the scan line (108) applies a voltage to the gates (107 a and 107 b) ofthe TFTs (104 a and 104 b) on that row of cells which turn on the TFTs.At the same time, signals for each electrode are applied at signal lines(106 a and 106 b), which are connected to the sources (105 a and 105 b)of the TFTs. These signals are then switched to the drains (103 a and103 b) of the TFTs, which are connected to the bottom electrode andin-plane electrodes respectively. The signals form the desired biascondition of each cell. Storage capacitors (109 a and 109 b) are addedto hold the voltages, so the voltages at the electrodes continueproviding the bias to the cell even after the switching. This drivingscheme speeds up the switching time dramatically. In addition, after thestorage capacitors (109 a and 109 b) are charged, the driver cancontinue switching the next row. The switching time for each row ofelectrodes is only the charging time for the storage capacitors. Thisgreatly reduces the response time of the display.

(2) Driving Timing Sequences

To illustrate the dual mode switching of this invention, electrophoreticcells comprising a common black background and positively charged whiteparticles dispersed in a clear colored solvent are used.

In a preferred embodiment, the top electrode of the device ispermanently connected to ground (0V). As illustrated in FIG. 10C, thescan line is set at a voltage, Von, to turn on all the TFTs on thescanning row. Voltages at the signal lines are then switched to thebottom electrode and the in-plane electrodes. When the bottom electrodeand in-plane electrodes are both set at a negative voltage, particles inthe cell move to the bottom surface of the cell. The bottom of the cellwill be covered by the white particles and the color of the dielectricsolvent (i.e., red, green or blue) is seen through the top transparentconductor layer.

As illustrated in FIG. 10D, when the bottom electrode and in-planeelectrodes are both set at a positive voltage, particles in the cellmove to the top surface of the cell. When the white particles in thecell migrate to the top of the cell, the resulting white color is seenfrom the top through the conductor layer.

As illustrated in FIG. 10E, when the bottom electrode is set at 0V, andthe in-plane electrodes are set at a negative voltage, particles move tothe area of the small in-plane electrodes, resulting in the blackbackground color being seen through the top transparent conductor film.

After a row of cells are scanned, the storage capacitors of each cellare charged according to the signal line voltage. When a row of cellsare not being scanned, it is in the non-scan phase. During the non-scanphase, the electrode with a voltage set by the storage capacitorcontinues to generate the electric field and control the movement ofparticles.

The scan time limit of this design is determined by the storagecapacitor discharge time. The capacitor needs to be refreshed(recharged) before the voltage drops more than 10% to maintain a goodbias voltage. The display response (on/off) time is determined by thepixel on/off time, because each pixel can be quickly charged andrefreshed. Thus, the line to line scan delay can be eliminated.

IV(c) Combination of Passive Matrix and TFT Active Matrix

(1) Electrode Circuit Design

In this novel design, both the passive matrix electrodes and the TFTactive matrix are used in the driving electrode design. FIG. 11A showsthe top view and FIG. 11B shows the side view of the driving circuitdesign. The bottom electrode layer (117) comprises the bottom columnelectrode (111) and in-plane electrodes (112). The in-plane electrodesof each cell is connected to the drain of a TFT (113). The source of theTFT is connected to a signal line (114) which runs vertically throughthe bottom surface of the device. The gate of the TFT is connected to ascan line (115) which runs horizontally through the bottom surface ofthe device. The scan and signal lines form a matrix structure, but thetwo types of lines are insulated from each other. The cells (110) aresandwiched between one top layer (116) and a bottom layer (117). Thehorizontal bars are the top row electrodes (118) that are transparentand run through the top of the cells (110). There is a gap (119)separating the two in-plane electrodes (112) and the column electrode(111). The cross section of the top row electrode, the bottom columnelectrode, and the in-plane electrodes define the display cell.

The top row electrodes (118) and bottom column electrodes (111) form thepassive matrix which controls the movement of particles in the up anddown direction. The in-plane electrodes (112) and the bottom columnelectrode (111) provide the bias voltage for the in-plane movement ofthe particles. The in-plane electrodes are controlled by the TFT activematrix.

(2) Driving Timing Sequences

To illustrate the dual mode switching of this invention, electrophoreticcells comprising a common black background and positively charged whiteparticles dispersed in a clear colored solvent are used.

As illustrated in FIG. 11C, the scan signal is set at a voltage, Von, toturn on all the TFTs on the scanning row. Voltages at the signal linesare then switched to the in-plane electrodes. When the bottom columnelectrode and in-plane electrodes are both set at a low voltage and thetop row electrode is set at a high voltage, particles in the cell moveto the bottom surface of the cell. The bottom of the cells consequentlyis covered by the white particles and the color of the dielectricsolvent (i.e., red, green or blue) is seen through the top transparentconductor layer.

As illustrated in FIG. 11D, when the bottom electrode and in-planeelectrodes are both set at a high voltage and the top row electrode isset at a low voltage, particles in the cell move to the top surface ofthe cell. When the white particles in the cell migrate to the top of thecell, the white color is seen from the top and the cell is in the whitestate.

As illustrated in FIG. 11E, when the top row electrode and bottom columnelectrode are set at a high voltage and the in-plane electrodes are setat a low voltage, particles move to the area of the small in-planeelectrodes, resulting in the black background color being seen throughthe top transparent conductor film.

This design has the advantage of reducing the in-plane switching time.As the in-plane electrodes are controlled by the TFTs, the storagecapacitor holds the voltage on the in-plane electrodes. During thenon-scan phase, the electrode with a voltage set by the storagecapacitor continues to generate the electric field and control themovement of particles. This effectively reduces the cell switching time.

IV(d) Alternative Combination of Passive Matrix and TFT Active Matrix

(1) Electrode Circuit Design

In another novel design, both the passive matrix electrodes and TFTactive matrix are used in the driving electrode design. FIG. 12A showsthe top view and FIG. 12B shows the side view of the driving circuitdesign. The bottom electrode layer (127) comprises bottom columnelectrode (121) and in-plane electrode (122). The bottom electrode ofeach cell connects to the drain of a TFT (123). The source of the TFTconnects to a signal line (124), which runs vertically through thebottom surface of the device. The gate of the TFT connects to a scanline (125), which runs horizontally through the bottom surface of thedevice. The scan and signal lines form a matrix structure, but they areinsulated from each other. The cells (120) are sandwiched between a toplayer (126) and a bottom layer (127). The horizontal bars are the toprow electrodes (128) that are transparent and run through the top of thecells. There is a gap (129) separating the two in-plane electrodes (122)and the bottom electrode (121). The cross section of the top rowelectrode, the bottom electrode, and the in-plane electrodes define thedisplay cell.

The top row electrodes and the in-plane electrodes form the passivematrix. The in-plane electrodes and the bottom electrode provide thebias voltage for the in-plane movement of the particles. The bottomelectrodes are controlled by the TFT active matrix.

(2) Driving Timing Sequences

To illustrate the dual mode switching of this invention, electrophoreticcells comprising a common black background and positively charged whiteparticles dispersed in a clear colored solvent are used.

As illustrated in FIG. 12C, the scan signal is set at a voltage, Von, toturn on all the TFTs on the scanning row. Voltages at the signal linesare then switched to the bottom electrode. When the bottom electrode andin-plane electrodes are both set at a low voltage, and the top rowelectrode is set at a high voltage, particles in the cell move to thebottom surface of the cell. The bottom of the cells will be covered bythe white particles and the color of the dielectric solvent (i.e., red,green or blue) is seen through the top transparent conductor layer.

As illustrated in FIG. 12D, when the bottom electrode and in-planeelectrodes are both set at a high voltage, and the top row electrode isset at a low voltage, particles in the cell move to the top surface ofthe cell. When the white particles in the cell migrate to the top of thecell, the resulting white color is seen from the top and the pixel is inthe white state.

As illustrated in FIG. 12E, when the top row electrode and bottom columnelectrode are set at a high voltage, and the in-plane electrodes are setat a low voltage, particles move to the area of the small in-planeelectrodes, resulting in the black background color being seen throughthe top transparent conductor film.

This design has the advantage of reducing the up-down switching time. Asthe bottom electrode is controlled by the TFTs, the storage capacitorholds the voltage on the bottom electrode. During the non-scan phase,the electrode with a voltage set by the storage capacitor continues togenerate the electric field and control the movement of the particles.This effectively reduces the cell switching time.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective, spirit and scope of the present invention.All such modifications are intended to be within the scope of the claimsappended hereto.

It is therefore wished that this invention to be defined by the scope ofthe appended claims as broadly as the prior art will permit, and in viewof the specification.

1. An electrophoretic display comprising an array of electrophoreticcells wherein each of said electrophoretic cells comprises a) a toplayer comprising a top electrode, b) a bottom layer comprising a bottomelectrode and at least one in-plane electrode, c) opaque partitionwalls, d) a cell background, and e) an electrophoretic compositionfilled therein which electrophoretic composition comprises chargedpigment particles of the same color and carrying the same chargepolarity dispersed in a dyed dielectric solvent or solvent mixture,whereby the charged pigment particles are capable of moving in avertical or planar direction.
 2. The electrophoretic display of claim 1wherein said opaque partition walls are white.
 3. The electrophoreticdisplay of claim 1 wherein said opaque partition walls are gray.
 4. Theelectrophoretic display of claim 1 wherein said opaque partition wallsare formed from a composition comprising a UV curable material and afiller, dye or pigment.
 5. The electrophoretic display of claim 4wherein said filler is selected from the group consisting of silica,ZnO, TiO₂, BaSO₄, CaCO₃ and polymer particles.
 6. The electrophoreticdisplay of claim 4 wherein said filler is silica or polymer particles.7. The electrophoretic display of claim 4 wherein said filler is in theamount of 1–20% by weight of the composition.
 8. The electrophoreticdisplay of claim 7 wherein said filler is in the amount of 2–10% byweight of the composition.
 9. The electrophoretic display of claim 1wherein said opaque partition walls are formed from a compositioncomprising a radiation curable material and a latent light scatteringmaterial.
 10. The electrophoretic display of claim 9 wherein said latentlight scattering material is a material that is compatible with theradiation curable material before curing but becomes incompatible aftercuring.
 11. The electrophoretic display of claim 9 wherein said latentlight scattering material is a thermally or photochemically triggeredgas-releasing material.
 12. The electrophoretic display of claim 11wherein said thermally or photochemically triggered gas-releasingmaterial is selected from the group consisting of CO₂ producingcarboxylic acids, bicyclic lactones or heterocycles; CO producingketones or bicyclic adducts; SO and SO₂ producing sulfones, sulfonyloxycompounds or heterocycles; N₂ producing azoalkanes, azides,diazomethanes, N-nitroso compounds, diazo, triazo or tetraazoheterocycles; COS producing xanthates and O₂ producing endoperoxides.13. The electrophoretic display of claim 11 wherein said thermally orphotochemically triggered gas-releasing material is a phenol, tropolone,pyridine, pyrazine, pyrrole or a halogenated derivative thereof.
 14. Theelectrophoretic display of claim 1 wherein said opaque partition wallsare colored.
 15. The electrophoretic display of claim 1 wherein theinside surface of the partition walls of an electrophoretic cell is ofthe same color as that of the electrophoretic composition in saidelectrophoretic cell.
 16. The electrophoretic display of claim 15wherein the two sides of a partition wall are of different colors. 17.The electrophoretic display of claim 1 wherein each of saidelectrophoretic cells displays the color of the charged pigmentparticles, the color of the dielectric solvent or solvent mixture, orthe color of the cell background.
 18. The electrophoretic display ofclaim 1 wherein each of said electrophoretic cells is sealed with asealing layer formed from a sealing composition having a specificgravity lower than that of the electrophoretic composition.
 19. Theelectrophoretic display of claim 1 wherein each of said electrophoreticcells is sealed with a sealing layer which is hardened in situ.